The present invention relates to improvements to semiconductor devices of the field effect transistor type and more specifically relates to those with a high cut-off frequency.
A field effect semiconductor device has been described in which a semi-insulating substrate supports the source and drain regions on the one hand and an active GaAs layer and and Al.sub.x Ga.sub.1-x As grid forming a heterojunction with the active layer on the other hand. In this type of transistor x is between 0.1 and 0.8 and to simplify both text and drawings Al.sub.x Ga.sub.1-x As is replaced by AlGaAs.
This device has been produced in several forms, depending on whether the active layer is of:
N-type weakly doped GaAs, PA0 P-type weakly doped GaAs, PA0 AlGaAs covered or not covered with an oxide layer.
or whether the grid is of:
All these constructions have the following common characteristics:
a high mobility electron layer in the heterojunction interface zone on the GaAs side,
the electron concentration of this layer is controlled by the polarization of the AlGaAs grid, which may or may not be covered by oxide.
The advantage of these devices compared with known field effect transistors is due to the great mobility of the electrons in the interface layer--metal Schottky field effect transistor and MOS (metal/oxide/silicon) transistor with depletion or inversion which have a reduced electron mobility. However, there are limitations on the load control by the AlGaAs grid.
Firstly the controlled load, i.e. the current between the source and drain I.sub.DS varies with the square root of the voltage applied to the grid V.sub.G. In other words the characteristics I.sub.DS /V.sub.G are not linear. These non-linear characteristics also exist in MESFET or MOSFET transistors. This is not a serious disadvantage, but from the use standpoint it is more advantageous to make them linear. For this reason in MESFET transistors linearization is sought and sometimes obtained in a relatively complicated manner by the formation of an active layer with variable doping profile.
Moreover, the concept of the load control by the AlGaAs grid leads to special transistor construction differing from those of conventional MESFET transistors, particularly due to the technology of the grid, drain and source contacts. Thus, their manufacture requires a different technology from that of GaAs MESFET, making their industrial production more difficult.